"The surprising thing is that it hasn't happened here," Livingstone says. "There will always be some people out there that are disgruntled or alienated from the societies they live in that are easy prey to people to come in and try to recruit them. They tend to be young people, the same kind of profile of people that would join a cult or something like that. And they can get them to strap on a bomb belt or backpack and walk into a crowded place.
"We're clearly as vulnerable as London, and we're, frankly, a target more in the crosshairs. They would like to bring the war back to what they feel is the belly of the beast, and they haven't been successful in doing so yet.
"But I think most observers in the law enforcement and intelligence community believe that it's no longer a question of if, but it's only a question of when, because we have the same vulnerabilities that they have in Great Britain."
Livingstone told co-anchor Hannah Storm Thursday the latest incidents send a "" from the terrorists that they won't be deterred. He tells Smith the same group seems to be behind Thursday's incidents in London as the deadly attacks earlier this month: "There are some reports that the bombs were almost identical, or maybe identical, to the bombs that exploded two weeks ago.
"London was traumatized yesterday, but nothing like it would have been if, indeed, you had seen another 50 to 100 people killed and another 700 or 800 people wounded.
"This was a great stroke of good fortune, and they have the devices now, as well as (having) one of the (suspected) suicide bombers, so that should give them plenty of evidence to start rolling up this network."
Livingstone adds he thinks steps such as New York City's random searches of backpacks and containers carried by would-be subway passengers may do more to increase riders' confidence than to actually improve security.
"The mass transit and light rail systems in the United States are so massive," he observes, "with so many people getting on and off, often for just one or two stations. It's an impossible thing to do to adequately protect them without more draconian measures."